Field of Invention
The present invention relates to an isolated power supply circuit with a programmable function and a control method thereof; particularly, it relates to such isolated power supply circuit and control method thereof with a relatively shorter output level transition period.
Description of Related Art
FIG. 1 shows a schematic diagram of a conventional isolated power supply circuit 100. As shown in FIG. 1, a rectifier circuit 101 rectifies an AC voltage Vac to generate an input voltage Vin. The rectifier circuit 101 is for example a bridge rectifier circuit. A transformer circuit 102 of the isolated power supply circuit 100 receives the input voltage Vin, and converts it to an output voltage Vo. The isolated power supply circuit 100 includes the aforementioned transformer circuit 102, a power switch circuit 103, a control circuit 105, an input current sense circuit 106, and an output voltage sense circuit 107. The control circuit 105 generates an operation signal GATE according to a current sense signal CS generated by the input current sense circuit 106 and a feedback signal FB generated by the output voltage sense circuit 107, for converting the input voltage Vin to the output voltage Vo. The transformer circuit 102 includes a primary winding W1 and a secondary winding W2. The secondary winding w2 is electrically connected to a reference level REF, and the primary winding W1 and a third winding W3 are coupled to a ground level GND. The output voltage sense circuit 107 senses the output voltage Vo by the third winding W3 to generate the feedback signal FB, wherein voltage across the third winding W3 is proportional to the voltage across the secondary winding W2, i.e., the output voltage Vo. For better logic in description, the third winding W3 is described as a part of the output voltage sense circuit 107; in circuit implementation, the third winding W3 may be included as a part of the transformer circuit 102.
The aforementioned conventional isolated power supply circuit 100 can only provide the output voltage Vo at one single predetermined level. The conventional isolated power supply circuit 100 can not provide different output voltages Vo at different levels according to different loading requirements. There is another prior art isolated power supply circuit which provides a programmable function wherein the output voltage Vo can switch between different levels, but this prior art requires a long transition period when the output voltage Vo switches from one level to another.
In view of the above, the present invention proposes an isolated power supply circuit with a programmable function and a control method thereof, whereby a transition period between different output voltage levels is shortened.